Most rk3288-based boards are derived from the EVB and thus use a PWM
regulator for the logic rail.  However, most rk3288-based boards don't
specify the PWM regulator in their device tree.  We'll deal with that
by making it critical.

NOTE: it's important to make it critical and not just IGNORE_UNUSED
because all PWMs in the system share the same clock.  We don't want
another PWM user to turn the clock on and off and kill the logic rail.

This change is in preparation for actually having the PWMs in the
rk3288 device tree actually point to the proper PWM clock.  Up until
now they've all pointed to the clock for the old IP block and they've
all worked due to the fact that rkpwm was IGNORE_UNUSED and that the
clock rates for both clocks were the same.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>